The invention relates to a method of manufacturing a composite material comprising a substrate and, disposed thereon, a layer consisting essentially of pure sp.sup.2 and sp.sup.3 hybridized carbon wherein the sp.sup.3 hybridized carbon content increases with increasing distance from the substrate.
Such composite material structures and methods of making them are disclosed in a publication by S. J. Bull: "Tribology of carbon coatings: DLC, Diamond and Beyond", Diamond and Related Materials 4 (1995), pages 827-836. The publication is concerned with amorphous carbon layers deposited on substrates which carbon layers include sp.sup.2 as well as sp.sup.3 bonds: Such carbon (DLC) layers are called "diamond-like carbon" (DLC) layers. The hardness of such layers depends on various parameters, particularly on the proportion of the sp.sup.3 hybridized bonds or respectively, the bias voltage applied during the manufacture of the layers in a sputtering procedure. However, adhesion of the layer to the substrate is considered by the author of the publication to be problematic. Since diamond-like layers are chemically inert, they do not form bonds with the substrate. For this reason, frequently a carbide forming element is deposited on the substrate as an intermediate layer which provides for the best adherence to diamond-like layers. Silicon or silicon carbide is suitable for such an intermediate layer. Reference is made in the publication to intermediate layers of complicated composition. Another problem resides in the fact that the greatest hardness is obtained with those diamond-like layers which have the highest internal tensions.
In the Proceedings of the 2.sup.nd International Conference on the Applications of Diamond Fibers and Related Materials, Editors M. Yoshikawa et al., pages 221 to 228 (Richard L. C. Wu and Kazuhisa Miyoshi: "Application of Diamond and Diamond-like Carbon Films as Lubricating Coatings") MYU, Tokyo 1993, diamond-like layers are also disclosed which were applied to a substrate ([100] Si) by direct ion beam deposition from a Kaufman ion source. The authors state that the proportion of the sp.sup.3 -hybridized carbon was about 50 times as large as the sp.sup.2 hybridized carbon proportion. The layers were found to be very hard and amorphous.
In the same Proceedings, there is furthermore a publication by Erich Bergmann: "Industrial Applications of Metal Containing Diamond Like Carbon Fibers" (pages 833-844). The author compares pure diamond-like carbon layers with metal containing diamond-like carbon layers and arrives at the conclusion that, with the metal-containing layers, among others, the problem of adherence of the layers on the substrate can be avoided.
It is the object of the present invention to provide a method of manufacturing a composite material structure including a surface with a relatively high sp.sup.3 hybridized carbon content which is therefore very hard, but which adheres well to the substrate and, inspite of the high hardness, has relatively low internal stresses.